In recent years, the domestic and international researchers and producers in the field of refining catalysts has devoted themselves to improve the performance of FCC catalyst by means of molecular sieves with various types of structure in order to increase the yields of light olefins in FCC plants. This is because the primary process for traditional production of ethylene and propylene, i.e. steam pyrolysis, is performed by the free radical reactions, wherein the temperature is high and the requirement to feedstock is rigorous. In contrast, catalytic pyrolysis for producing light olefins has relatively lower cost and is the hot spot in the domestic and international research for increasing light olefin capacity. Functionalized shape-selective molecular sieves deserve wide attention as the major active constituent in such kind of catalytic converting catalyst.
Since USA Mobil Company has developed the ZSM-5 zeolite molecular sieve (U.S. Pat. No. 3,702,886) in 1972, it has been widely applied in petrochemical processes, such as shape-selective cracking (CN 1872415A), alkylation, isomerisation, disproportionation, catalytic dewaxing, etherification and etc. of hydrocarbons, due to its properties of relatively high silica to alumina ratio, unique pore structure, and excellent thermal and hydrothermal stabilities. In particular, adding ZSM-5 zeolite into normal catalysts or aids for catalytic cracking enables to significantly increase the yields of light olefins and the octane number (U.S. Pat. No. 5,997,728).
In the composition of modern FCC catalysts, the molecular sieve active constituent may significantly affect the comprehensive reaction performance of the catalyst. In general, besides a portion of shape-selective zeolite molecular sieve ZSM-5, a FCC catalyst further contains 30-40% by weight of Y type zeolite, which comprises REY, USY, REUSY and REHY, and other types of molecular sieves as the primary active constituent in the FCC catalyst. However, the sodium content in Y type zeolites are much more higher than that in H-ZSM-5 molecular sieves, while the active centers of the H-ZSM-5 molecular sieve are very readily to be covered by sodium ions via ion exchange in the presence of sodium ions. Therefore, during the preparation of FCC catalyst, the H-ZSM-5 molecular sieve is very readily to be contaminated by sodium ions from the environment and thus deactivated, and thus its catalytic cracking activity and selectivity will be affected. The research about increasing the sodium-resisting contamination activity of ZSM-5 molecular sieves has not been reported at present.
At present, a number of researches have been performed about various methods for modifying phosphorus-modified ZSM-5 zeolite molecular sieves, which are mainly applied in petrochemical processes, such as shape-selective cracking, alkylation, isomerisation, disproportionation, catalytic dewaxing, etherification and etc. of hydrocarbons.
In U.S. Pat. No. 4,399,059, diammonium hydrogen phosphate or ammonium dihydrogen phosphate is mixed with NH4-ZSM-5 and dried, and the mixture is calcined at 500° C. so as to produce a phosphorus-modified ZSM-5 zeolite, which makes it possible to significantly improve the selectivity of para isomer product when used in the isomerisation reaction of xylene.
U.S. Pat. No. 5,171,921 discloses a ZSM-5 molecular sieve modified by impregnating with phosphorus compounds. Such modified molecular sieve may be used as the catalytically active constituent that converts olefins or aliphatic hydrocarbons into C2-C5 olefins.
U.S. Pat. No. 3,972,382 and U.S. Pat. No. 3,965,208 disclose that the reaction selectivity of the HZSM-5 is improved after being modified with trimethyl phosphite.
CN 85102828 reports modifying the ZSM-5 molecular sieve using a method by impregnating and evaporating, and the phosphorus-modified molecular sieve has a substantially improved activity in the shape-selective catalysis for preparing para-ethyltoluene by alkylation of toluene with ethylene.
CN 97120271 reports a phosphorus-containing faujasite as hydrocarbon cracking catalyst, said phosphorus-containing faujasite is prepared by uniformly mixing faujasite with an aqueous solution of a phosphorus-containing compound followed by drying and calcining at 450-600° C. for more than 0.5 h, and has a relatively good catalytic activity.
However, according to the research about the sodium-resisting contamination activity of H-ZSM-5 zeolite molecular sieves after conventional phosphorus modification, it has been founded that the molecular sieve after conventional phosphorus modification will loss plenty of modifying constituent in the molecular sieve, and a number of sodium ions will exist on surface of the molecular sieve via ion exchange, that is, it does not possess sodium-resisting contamination activity.